"How does carbon enter water?" : Carbon dioxide from the atmosphere dissolves in the surface waters of the ocean. Some of the carbon dioxide stays as dissolved gas, but much of it gets turned into other things. Photosynthesis by tiny marine plants (phytoplankton) in the sunlit surface waters turns the carbon into organic matter.
"How does aquatic plants get carbon" : The only difference between photosynthesis in aquatic and land plants is where in their environments they get these nutrients. Land plants get water from the ground through their extensive root system, carbon dioxide from the air through their stomata (tiny holes in a plant's leaves), and energy from the sun.
